The free pass over Windows RT, however, eliminates an investigation that would in many ways be a repeat of the 2009 exchange between the Commission and Microsoft.

"We have looked at Windows RT, and on the basis of our investigation so far, there are no grounds to pursue further investigation on this particular issue," Joaquin Almunia, the EU's head antitrust official, said at a news conference Wednesday. "But we will closely monitor all the elements of the Windows software and how Microsoft complies to [its] commitments."

Almunia's agency fielded complaints last summer that accused Microsoft of stymying other browser makers' efforts to build software that runs in Windows RT, the offshoot of Windows 8 designed for ARM-powered tablets.

Microsoft will launch its first-ever computing device, the "Surface RT" tablet, on Friday.

Although the Commission has declined to name the complainants -- and did not do so yesterday -- the most vocal about Windows RT has been Mozilla, the open-source developer whose Firefox is the world's second-most-popular Web browser.

Mozilla is particularly strong in Europe, where it has an estimated 29% of the market, behind only Google's Chrome. Its European share is considerably higher than the 18% it enjoys in the U.S., according to Irish metrics company StatCounter.

In July, Reuters reported that regulators were looking into allegations that Microsoft refused to give rivals full access to APIs (application programming interface) in Windows RT.

It wasn't a surprise: The Windows RT APIs had been a source of tension between Microsoft on one hand, and Mozilla and Google on the other, for months.

Mozilla noted that Microsoft had given full API access only to the latter's own Internet Explorer 10 (IE10), the sole browser that will run on Windows RT's conventional desktop. Sans those APIs, it was impossible to create a viable browser for Windows RT, Mozilla argued.

"We could build a beautiful Firefox that looked really nice on Metro [the then-current code name for the Windows RT user interface, or UI], but Firefox would be so crippled in terms of power and speed that it's probably not worth it to even bother," said Asa Dotzler, director of Firefox, in May.

He was blunt in his assessment. "Microsoft is trying to lock out competing browsers when it comes to Windows running on ARM chips," Dotzler said, referring to Windows RT.

Google said it "share[ed] the concerns Mozilla has raised."

Both Mozilla and Google are working on browsers that run in the "Modern," formerly "Metro," UI on Windows 8, a more robust operating system destined for traditional PCs as well as some tablets. The Modern UI on Windows 8 is identical to Windows RT's interface, and apps written for the former are supposed to work in the latter.

It's uncertain whether the pair will submit their Modern apps to the Windows Store, the Microsoft-curated online outlet that is the sole source of Windows RT software.

The European Commission's decision not to pursue complaints about Windows RT drew a carefully-worded comment from Mozilla on Wednesday. "We hope the European Commission is correct in its conclusions and users are not deprived of browser choice in Window RT operating systems," said Harvey Anderson, Mozilla's general counsel, in an email.

Google did not reply to a request for comment on Almunia's decision.

Along with the pass given to Windows RT, the Commission also said that Windows 8 must include the browser choice screen that Microsoft agreed to in 2009.

"We have raised issues to Microsoft relating to Windows 8, which is to be released soon," Almunia said. "If a user decides to set a rival browser as the default browser, there should not be unnecessary warning windows or confirmations by the user, and the Internet Explorer icon should also be unpinned from the Start screen."

Windows 8 initially sets IE10 as the default browser in both the Modern and traditional UIs, the OS's two different -- and some say jarring -- modes. In other words, only one Modern browser may run at a time.

Previously, the Commission had made it clear that Windows 8 must abide by the 2009 deal.

Microsoft quickly acquiesced to Almunia demands.

"After discussions with the Commission, we are changing some aspects of the way the Browser Choice Screen works on Windows 8 and will have those changes implemented when Windows 8 launches later this week," Microsoft said in a statement. Presumably, the changes will be pushed to Windows 8 hardware via Windows Update.

Microsoft could face massive fines for its failure to provide European users of Windows 7 with the browser ballot from February 2011 to July 2012.

Yesterday, Almunia again used strong language to describe Microsoft's violation of the 2009 agreement. "If companies enter into commitments, they must do what they have committed to do or face the consequences," Almunia said. "Companies should be deterred from any temptation to renege on their promises or even to neglect their duties. This is why, when this happens, the Commission has the power to impose fines."

Later, he added, "This is a very serious message [we are sending], not to infringe the commitments."

Last summer, Microsoft quickly admitted the infraction, then updated Windows 7 so that all required users got the browser ballot. It also claimed that the omission had been a "technical error."

The Commission has given Microsoft four weeks to reply to the charges, which in the EU are called "statement of objections;" the American company can also request an oral hearing.

Under EU law, the Commission can levy fines as high as 10% of a company's annual revenue, putting Microsoft's potential liability at an astounding $8.9 billion. Microsoft's global revenue for the five quarters starting Jan. 1, 2009, totaled $89.5 billion.

Almunia took a question Wednesday about possible fines, but declined to get specific. "We are not yet at this step in our procedure ... [and] we will listen very carefully to Microsoft, how they answer the statement of objections, and if our objections are confirmed we will go to the next step in the procedure, that is to estimate what will be the adequate fines," Almunia said.

This summer, he noted that Microsoft's violation was the first time that a company had reneged on binding promises. "Non-compliance with an Article 9 commitment has never occurred in the past," said Almunia at the time.

In June 2009, near the end of Windows 7's development, Microsoft said it would ship the operating system without Internet Explorer 8 (IE8) to European customers to sidestep pressure from the Commission, which was reacting to an earlier complaint filed by Norway's Opera Software.